The invention relates to a device for controlling the gear ratio of a continuously variable transmission in a vehicle having a prime mover and an output governor device. To them are specified, via an accelerator pedal, theoretical rotational speed values of the prime mover. The theoretical rotational speed values are compared with measured actual rotational speed values. The device also has a setting device for changing the gear ratio of the continuously variable transmission. Control signals of a rotational speed governor are fed to it which result from the differences between theoretical and actual rotational speed values of the prime mover.
As a rule, the above mentioned continuously variable transmissions have bevel discs which, between them, enclose a rotary traction means. Axial contact pressures are exerted upon the bevel discs via a hydraulically working setting device.
The position of the throttle valve, or the like, serves as an input parameter for control of a continuously variable transmission via a control part (gas or accelerator pedal). For said input parameter, a theoretical rotational speed value, which is compared with the actual rotational speed value, is preset for the prime mover. Control divergences release the adjustment of the transmission, for example, by a solenoid valve. When the rotational speed is too low, the transmission ratio becomes smaller, when the rotational speed is high, it is adjusted larger. The control loop is closed by a repeated comparison with the actual rotational speed value.
In the operational strategies known already, the point of departure is that by the position of the control part the driver presets a desired performance from which results a desired engine rotational speed nM-theor, optionally under the influence of optimization criteria (mode of driving, driving environment). An optimization criterion can be, for example, the intersection point of a line of minimal fuel consumption with the actual line of the throttle valve position in the engine performance graph. The appertaining transmission ratio of the continuously variable transmission iV-theor. then results as a sequential parameter from the desired engine rotational speed nM-theor. and the actual vehicle speed vF. By using an electronic transmission control, it is possible to select different driving programs (as a rule economical or performance oriented) by a switch which can be situated, for example, on the selector lever.
Depending on which driving program is active, different reaction speeds, according to the parameter of the change in position of the throttle valve or the adjusted speed, are to be expected or taken into consideration.
The inertia moments of the transmission and the engine are essential for the demands on the hydraulic system, kinematics and the time characteristic of the system. This means, among other things, that too quick a shift during a down change leads to braking of the vehicle, since all of the rotating masses of the vehicle inertia must be accelerated and that no increase in speed is to be registered during the shift, since the engine torque is fully used for acceleration of the rotating masses of the transmission of the engine. This fact makes itself disadvantageously noticeable in certain driving conditions. Thus, for example, when the driver expects an instantaneous acceleration during a down change. In order to ensure a continuous speed increase, the adjusting speed, that is, the change of transmission ratio of the continuously variable transmission, must be variable over the time. The timed change of the adjustment is here of basic significance for the quality of the control and thus finally also for the driving comfort obtainable.
DE-A 36 36 463 proposes a method for limiting the variation speed. Such limitation results essentially for reasons of comfort, since torque jumps and undesired vehicle accelerations or decelerations must be avoided. There must also be brought about, as direct as possible, a relation between the position of the control member (accelerator pedal) and the driving impression (improvement of the subjective driving impression, driveability).
EP-A 0 364 270 has disclosed a control system for a continuously variable transmission in which sensors are provided for detecting the rotational speeds of the bevel discs and of the degree of opening of the throttle valve. A theoretical transmission ratio is calculated from the throttle valve opening. The variation gradient, with which the transmission ratio of the continuously variable transmission is produced, is corrected according to the actual driving speed. At low vehicle speeds, the variation is relatively quick while, at high vehicle speeds, the variation gradient (diV/dt) is reduced.
Finally, there has been disclosed in DE-A 42 29 585 a device for control of the gear ratio of a continuously variable transmission in which a stationary theoretical rotation speed value is deduced from an operational performance graph and fed to a set-value filter, at the exit, which transmits an unsteady theoretical rotational speed value. The set-value filter affects the dynamic characteristic. A specific characteristic, of the variation characteristics, is obtained by adequate transmission functions.
The proposals belonging to the prior art all seek to obviate one disadvantage of a continuously variable transmission: in special situations the continuously variable transmission reacts with an uneven acceleration characteristic. The driveability can be objectively made clear by the following characteristic: upon a sudden change in the position of the throttle valve, the theoretical rotational speed value of the prime mover bounces vertically. Contrary to said significant rise in the rotational speed of the engine, the vehicle speed remains constant, at first, for a certain period of time. It is above all disadvantageous that the driver accordingly, at first, does not perceive any acceleration of the vehicle. Under unfavorable circumstances, that is, when the variation gradient has not been adapted to the transmission ratio, in addition, a brief deceleration of the vehicle occurs. Said behavior is not expected by the driver and therefore is felt as unpleasant. To complicate this is added that the driver, who via the accelerator pedal, has announced a need for power but, despite a significant rise of the engine rotational speed, experiences no acceleration of the vehicle, tries to clear this situation by announcing a still greater need for power. The consequence of this is an overcontrol of the throttle valve. In the reaction that sets in, said overcontrol leads to a discharge of the vehicle. It is these disadvantages that are meant when speaking of a so-called "suspenders effect" in connection with continuously variable transmissions.